1. Field of the Invention
This invention generally relates to an apparatus and method for increasing a slew rate of an operational amplifier, and more particularly to an apparatus and method for increasing a slew rate of an operational amplifier by using a monitoring control device controlled by the output stage to control the supplementary output device and the supplementary input device in order to increase a slew rate of an operational amplifier.
2. Description of Related Art
To acquire a higher slew rate of the operational amplifier (OPAMP), the conventional methods include increasing the current of the differential input pair or reducing the compensation capacitance. However, the former will increase the quiescent/operating current consumption; the latter will sacrifice the stability of the OPAMP. Another conventional art is to use the error amplifier to push the common source output stage, i.e., the push-pull output stage. I.e., it requires additional circuit to achieve the target. Although those conventional methods can increase the slew rate, they also have some drawbacks such as increase of the chip size, increase of the current, and sacrifice of the stability.
Referring to FIG. 1, FIG. 1 shows a conventional OPAMP 100 with a high slew rate. It includes an OPAMP A, two error amplifiers E1 and E2, and a push-pull output stage consisting of two MOSFETs M1 and M2. The error amplifiers E1 and E2 control these two MOSFETs M1 and M2 by connecting the inverting input terminal of the error amplifier to the output terminal 102 of OPAMP A, connecting the non-inverting input terminal of the error amplifier to the virtual short consisting of the node VO, and using a negative feedback loop consisting the error amplifier E1 and MOSFET M1, and the error amplifier E2 and MOSFET M2 to control the push-pull output stage consisting of MOSFETs M1 and M2, so as to provide the loading with the current for pushing or pulling.
The error amplifiers E1 and E2 can monitors whether the signals in the inverting input terminal and in the non-inverting input terminal are the same. If not, the MOSFET M1 or M2 will be turned on so that the push-pull output stage becomes a current source (i.e., push the current to the output terminal) or a current sink (i.e., pull the current to the output terminal or to the loading).
The operation principle of the OPAMP 100 is when the output voltage VO is lower than the output voltage V1 of the OPAMP A, the output voltage V2 of the error amplifier E1 will turn on the MOSFET M1 and the output voltage V3 of the error amplifier E2 will turn off the MOSFET M2; at the time the MOSFET M1 will push the current to the output terminal. When the output voltage VO is higher than the output voltage V1 of the OPAMP A, the output voltage V2 of the error amplifier E1 will turn off the MOSFET M1 and the output voltage V3 of the error amplifier E2 will turn on the MOSFET M2; at the time the MOSFET M2 will pull the current to the output terminal. When the output voltage VO is equal to the output voltage V1 of the OPAMP A, the output voltage V2 of the error amplifier E1 will make the MOSFET M1 operate under a static current and the output voltage V3 of the error amplifier E2 will make the MOSFET M2 operate under a static current; at the time the output voltage VO is equal to the output voltage V1 of the OMAMP A. I.e., when the input is equal to the output, the MOSFETs M1 and M2 will operate under the pre-set DC Bias condition.
The above structure generally is for heavy loading, e.g., small resistor and large capacitor. To make the MOSFETs M1 and M2 provide the loading with a high current, the aspect ratio of the MOSFETs M1 and M2 must be very large. Hence, the push-pull output stage requires a huge static current consumption. I.e., it is very difficult to achieve low power consumption and a high slew rate at the same time. Further, the circuit looks very simple, but practically is very complicated. If the error amplifiers E1 and E2 are a single stage amplifier, each at least requires 5 MOSFETs. Hence, the error amplifiers E1 and E2 totally at least require 10 MOSFETs.
In addition, to compensate the pole-zero, it requires 2 compensation capacitors if the Miller compensation is used. It also has to consider the offset voltage, the layout symmetry, bandwidth, and noise of the error amplifiers. Hence, those considerations inevitably increase the size of the chip and the production cost. Further, it also has to consider the influence of the cross distortion and the offset voltage VOS of the error amplifier to the linearity of the OPAMP. The above limitations make the above structure more complicated and occupy a huge size. Hence, although the conventional OPAMP 100 can provide an excellent driving ability and a simple structure, it occupies a huge size and increases the production cost. In addition, the issue of cross distortion, the problems generated from the characteristics of the error amplifier, and the issue of the high power consumption have to be considered.